1. Field of the Invention
This invention relates to a radiation image storage panel utilizing light emission characteristics of a stimulable phosphor.
2. Description of the Related Art
In lieu of conventional radiography, radiation image recording and reproducing techniques utilizing a stimulable phosphor have heretofore been used in practice. The radiation image recording and reproducing techniques are described in, for example, U.S. Pat. No. 4,239,968. The radiation image recording and reproducing techniques utilizes a radiation image storage panel (referred to also as the stimulable phosphor sheet) provided with a stimulable phosphor. With the radiation image recording and reproducing techniques, the stimulable phosphor of the radiation image storage panel is caused to absorb radiation, which carries image information of an object or which has been radiated out from a sample, and thereafter the stimulable phosphor is exposed to an electromagnetic wave (stimulating rays), such as visible light or infrared rays, which causes the stimulable phosphor to produce the fluorescence (i.e., to emit light) in proportion to the amount of energy stored thereon during its exposure to the radiation. The produced fluorescence (the emitted light) is photoelectrically detected to obtain an electric signal. The electric signal is then processed, and the processed electric signal is utilized for reproducing a visible image.
The radiation image recording and reproducing techniques have the advantages in that a radiation image containing a large amount of information can be obtained with a markedly lower dose of radiation than in the conventional radiography. Also, ordinarily, the radiation image recording and reproducing techniques are performed with built-in types of radiation image recording and read-out apparatuses comprising, built in a single apparatus, (a) means (recording means) for irradiating radiation, which carries image information, to the radiation image storage panel and thereby recording a radiation image on the radiation image storage panel, (b) means (read-out means) for exposing the radiation image storage panel, on which the radiation image has been stored, to the stimulating rays, which cause the radiation image storage panel to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, and photoelectrically detecting the emitted light to obtain an electric signal (an image signal), (c) means (erasing means) for irradiating erasing light to the radiation image storage panel, from which the image signal has been detected, and thereby erasing information remaining on the radiation image storage panel, and (d) a conveyance system, which connect the recording means, the read-out means, and the erasing means to one another and conveys the radiation image storage panel among them. The erased radiation image storage panel can be used again for the recording of a radiation image. Therefore, the radiation image storage panel is used repeatedly. Particularly, in the radiation image recording and read-out apparatuses described above, the radiation image storage panel is conveyed repeatedly, circulated, and used for the recording of a radiation image within the apparatus. Accordingly, the radiation image recording and reproducing techniques is efficient particularly for direct medical radiography, such as the X-ray image recording for medical diagnosis.
Ordinarily, the radiation image storage panel utilized for the radiation image recording and reproducing techniques has a basic structure comprising a substrate and a stimulable phosphor layer overlaid on one surface of the substrate. Ordinarily, the radiation image storage panel has a rectangular sheet-like shape. Also, ordinarily, a transparent protective film is formed on the surface of the stimulable phosphor layer, which surface is opposite to the surface that stands facing the substrate. The transparent protective layer protects the stimulable phosphor layer from chemical deterioration or physical shocks.
The stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed in the binder. The stimulable phosphor has the properties such that, when the stimulable phosphor is caused to absorb radiation, such as X-rays, and is then exposed to an electromagnetic wave (stimulating rays), such as visible light or infrared rays, the stimulable phosphor emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. Therefore, when the radiation image storage panel is exposed to the radiation, which carries image information of an object or which has been radiated out from a sample, the stimulable phosphor layer of the radiation image storage panel absorbs the radiation in proportion to the dose of radiation, and a radiation image of the object or the sample is stored as an image (a latent image) of energy from the radiation on the radiation image storage panel. The radiation image storage panel is then exposed to the electromagnetic wave, and the image having been stored on the radiation image storage panel can be detected as the light emitted by the radiation image storage panel. The emitted light is detected photoelectrically to obtain an image signal, the image signal is processed, and the thus obtained processed image signal can then be utilized for reproducing the radiation image of the object or the sample as a visible image.
As described above, the radiation image recording and reproducing techniques are the advantageous image forming techniques. However, as in the cases of an intensifying screen employed in the conventional radiography, it is desired that the radiation image storage panel utilized for the radiation image recording and reproducing techniques has a high sensitivity and can yield an image of good image quality (with respect to sharpness, graininess, and the like).
It is described in, for example, U.S. Pat. No. 4,346,295 and Japanese Unexamined Patent Publication No. 59(1984)-162499, that a radiation image storage panel capable of yielding an image of good image quality can be obtained by altering the ratio of a binder and a stimulable phosphor to each other with respect to the depth direction of the radiation image storage panel. Also, as a radiation image storage panel suitable for a technique for detecting light emitted from opposite surfaces of a radiation image storage panel and thereby detecting two image signals from the opposite surfaces of the radiation image storage panel, a radiation image storage panel has been proposed in, for example, Japanese Unexamined Patent Publication No. 7(1995)-287099, wherein the distribution of a binder and the distribution of ultramarine in a stimulable phosphor layer are biased to one side of the stimulable phosphor layer, such that the sharpness and graininess characteristics of an obtained radiation image may be improved.
As one of producing processes for obtaining the distribution described above, a producing process has been known, wherein a stimulable phosphor sublayer is formed on a temporary substrate having been applied with a releasing agent and is then separated from the temporary substrate, and a plurality of stimulable phosphor sublayers having thus been obtained are overlaid one upon another to form a stimulable phosphor layer. With the producing process, such that an image having good image quality can be obtained, a temporary substrate contact side surface of a stimulable phosphor sublayer, which surface was in contact with the temporary substrate when the stimulable phosphor sublayer was formed on the temporary substrate, and a temporary substrate contact side surface of an adjacent stimulable phosphor sublayer are adhered to each other. However, when the stimulable phosphor sublayer is formed on the temporary substrate, stimulable phosphor particles having a large specific gravity sink to the lower side, and the binder floats to the upper surface of the stimulable phosphor sublayer. As a result, at the temporary substrate contact side surface of the stimulable phosphor sublayer, the proportion of the binder becomes low. Therefore, when a plurality of stimulable phosphor sublayers are overlaid one upon another to form a stimulable phosphor layer, if the temporary substrate contact side surfaces of the stimulable phosphor sublayers, at which surfaces the proportions of the binder are low, are adhered to each other, the strength of adhesion between the stimulable phosphor sublayers will become low. Therefore, the problems occur in that, when the radiation image storage panel is conveyed repeatedly, the stimulable phosphor sublayers become separated from one another. Also, the problems occur in that the structure mottle characteristics become bad due to disturbance of the adhesion interface.
The primary object of the present invention is to provide a radiation image storage panel, which is provided with a stimulable phosphor layer composed of a plurality of stimulable phosphor sublayers overlaid one upon another, such that a sufficient adhesion strength may be obtained between the stimulable phosphor sublayers and disturbance at the stimulable phosphor sublayer interface may be suppressed, and which yields an image having good image quality and has good conveyance durability.
The present invention provides a radiation image storage panel, comprising at least a substrate and a stimulable phosphor layer overlaid upon the substrate, the stimulable phosphor layer being composed of a plurality of stimulable phosphor sublayers overlaid one upon another,
wherein at least two stimulable phosphor sublayers, which are adjacent to each other among the plurality of the stimulable phosphor sublayers overlaid one upon another, are overlaid one upon the other such that a temporary substrate contact side surface of one of the two adjacent stimulable phosphor sublayers, which surface was in contact with a temporary substrate utilized when the one adjacent stimulable phosphor sublayer was formed, and a temporary substrate contact side surface of the other adjacent stimulable phosphor sublayer, which surface was in contact with a temporary substrate utilized when the other adjacent stimulable phosphor sublayer was formed, stand facing each other, and
wherein a binder, which is contained in a top stimulable phosphor sublayer that is remotest from the substrate among the plurality of the stimulable phosphor sublayers overlaid one upon another on the substrate, contains a polyurethane binder, which has a weight-average molecular weight falling within the range of 200,000 to 500,000, in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer.
The polyurethane binder is a high-molecular weight compound having a urethane bond, xe2x80x94NHCOOxe2x80x94, in the repeating unit in the main chain. The polyurethane binder employed in the radiation image storage panel in accordance with the present invention has a weight-average molecular weight falling within the range of 200,000 to 500,000, and should preferably have a weight-average molecular weight falling within the range of 250,000 to 400,000. The polyurethane binder is contained in the top stimulable phosphor sublayer among the plurality of the stimulable phosphor sublayers overlaid one upon another on the substrate, i.e. in the stimulable phosphor sublayer which is remotest from the substrate. The proportion of the polyurethane binder contained in the top stimulable phosphor sublayer falls within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer, and should preferably fall within the range of 40% by weight to 80% by weight with respect to the total binder contained in the top stimulable phosphor sublayer.
For example, in cases where the number of the stimulable phosphor sublayers overlaid on the substrate is two, the two stimulable phosphor sublayers are overlaid one upon the other such that the temporary substrate contact side surface of one of the two stimulable phosphor sublayers, which surface was in contact with the temporary substrate utilized when the one stimulable phosphor sublayer was formed, and the temporary substrate contact side surface of the other stimulable phosphor sublayer, which surface was in contact with the temporary substrate utilized when the other stimulable phosphor sublayer was formed, stand facing each other. Also, the binder, which is contained in the top stimulable phosphor sublayer that is remoter from the substrate than the other stimulable phosphor sublayer is, contains the polyurethane binder, which has a weight-average molecular weight falling within the range of 200,000 to 500,000, in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer. In cases where the number of the stimulable phosphor sublayers overlaid on the substrate is three, two stimulable phosphor sublayers among the three stimulable phosphor sublayers should preferably be overlaid one upon the other such that the temporary substrate contact side surface of one of the two stimulable phosphor sublayers, which surface was in contact with the temporary substrate utilized when the one stimulable phosphor sublayer was formed, and the temporary substrate contact side surface of the other stimulable phosphor sublayer, which surface was in contact with the temporary substrate utilized when the other stimulable phosphor sublayer was formed, stand facing each other. The two stimulable phosphor sublayers are overlaid upon the substrate. Also, in such cases, a top stimulable phosphor sublayer containing the polyurethane binder, which has a weight-average molecular weight falling within the range of 200,000 to 500,000, in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer should preferably be overlaid upon the two stimulable phosphor sublayers, which have been overlaid upon the substrate in the manner described above.
In the radiation image storage panel in accordance with the present invention, sublayer thickness of each of the stimulable phosphor sublayers overlaid one upon another on the substrate should preferably be set such that the sublayer thickness of a stimulable phosphor sublayer remote from the substrate is smaller than the sublayer thickness of a stimulable phosphor sublayer close to the substrate.
As described above, the radiation image storage panel in accordance with the present invention comprises at least the substrate and the stimulable phosphor layer overlaid upon the substrate, the stimulable phosphor layer being composed of a plurality of stimulable phosphor sublayers overlaid one upon another. At least two stimulable phosphor sublayers, which are adjacent to each other among the plurality of the stimulable phosphor sublayers overlaid one upon another, are overlaid one upon the other such that the temporary substrate contact side surface of one of the two adjacent stimulable phosphor sublayers, which surface was in contact with the temporary substrate utilized when the one adjacent stimulable phosphor sublayer was formed, and the temporary substrate contact side surface of the other adjacent stimulable phosphor sublayer, which surface was in contact with the temporary substrate utilized when the other adjacent stimulable phosphor sublayer was formed, stand facing each other. Therefore, the radiation image storage panel in accordance with the present invention has a high permeability to light and can yield an image having good image quality with respect to sharpness and graininess. Also, ordinarily, when a stimulable phosphor sublayer is formed on a temporary substrate, stimulable phosphor particles having a large specific gravity sink to the lower side (the substrate side), and a binder floats to the upper surface of the stimulable phosphor sublayer. However, with the radiation image storage panel in accordance with the present invention, the binder, which is contained in the top stimulable phosphor sublayer that is remotest from the substrate among the plurality of the stimulable phosphor sublayers overlaid one upon another on the substrate, contains the polyurethane binder, which has a weight-average molecular weight falling within the range of 200,000 to 500,000, in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer. Therefore, when the top stimulable phosphor sublayer is formed on the temporary substrate, the binder does not readily float to the upper surface of the stimulable phosphor sublayer. Therefore, the adhesion strength between the top stimulable phosphor sublayer and the stimulable phosphor sublayer lying under the top stimulable phosphor sublayer can be enhanced, and the stimulable phosphor layer having a high resistance to sublayer separation can be obtained. Accordingly, the radiation image storage panel having an enhanced conveyance durability can be obtained. Specifically, an ordinary stimulable phosphor sublayer has a biased binder distribution such that, when the temporary substrate contact side surface of the stimulable phosphor sublayer, which surface was in contact with the temporary substrate when the stimulable phosphor sublayer was formed on the temporary substrate, and the opposite upper surface of the stimulable phosphor sublayer, which surface was not in contact with the temporary substrate when the stimulable phosphor sublayer was formed on the temporary substrate, are compared with each other, more of the binder is contained at the opposite upper surface, and less of the binder is contained at the temporary substrate contact side surface. However, with the radiation image storage panel in accordance with the present invention, wherein the binder, which is contained in the top stimulable phosphor sublayer that is remotest from the substrate among the plurality of the stimulable phosphor sublayers overlaid one upon another on the substrate, contains the polyurethane binder, which has a weight-average molecular weight falling within the range of 200,000 to 500,000, in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer, the top stimulable phosphor sublayer has little difference in binder distribution between the temporary substrate contact side surface and the opposite upper surface of the top stimulable phosphor sublayer. Therefore, the adhesion strength between the top stimulable phosphor sublayer and the adjacent stimulable phosphor sublayer can be enhanced markedly. Also, since the adhesion strength between adjacent stimulable phosphor sublayers can be enhanced, disturbance of the adhesion interface can be suppressed, and the problems can be prevented from occurring in that the structure mottle characteristics become bad due to disturbance of the adhesion interface.
With the radiation image storage panel in accordance with the present invention, wherein the proportion of the polyurethane binder contained in the top stimulable phosphor sublayer falls within the range of 40% by weight to 80% by weight with respect to the total binder contained in the top stimulable phosphor sublayer, the effects described above can be enhanced even further.
With the radiation image storage panel in accordance with the present invention, the sublayer thickness of each of the stimulable phosphor sublayers overlaid one upon another on the substrate may be set such that the sublayer thickness of a stimulable phosphor sublayer remote from the substrate is smaller than the sublayer thickness of a stimulable phosphor sublayer close to the substrate. Specifically, the thickness of the top stimulable phosphor sublayer, which is remotest from the substrate among the plurality of the stimulable phosphor sublayers overlaid one upon another on the substrate and which contains the polyurethane binder having a weight-average molecular weight falling within the range of 200,000 to 500,000 in a proportion falling within the range of 20% by weight to 90% by weight with respect to the total binder contained in the top stimulable phosphor sublayer, may be set to be thin. In such cases, the difference in binder distribution between the temporary substrate contact side surface and the opposite upper surface of the top stimulable phosphor sublayer can be minimized, and the image quality of the image obtained with the radiation image storage panel can be enhanced even further.
The present invention will hereinbelow be described in further detail with reference to the accompanying drawings.